Self-regulated drive apparatus for display systems

ABSTRACT

Apparatus addressing multi-position display devices directly from semiconductor integrated circuits and driving the display electrodes via passive coupling without exceeding the limitation on voltage excursions for the outputs of such circuits. The apparatus develops and applied to threshold-responsive display devices, including gas discharge display tubes or panels, only the amount of bias that is needed for operation of the device by the signal potentials that are available. Bias potential for the display electrodes is accumulated in steps until the device fires, and then is automatically adjusted each cycle in relation to the number of display electrodes that are being operated. The input signals are applied to one set of electrodes through coupling capacitors which are charged from a common bias capacitance during blanking intervals between the input pulses and are at least partially discharged during operation of the device. The bias potential for those electrodes is developed across the common capacitance by a regulator passively coupled to the cathodes by series feedback resistors which add charge to the bias capacitance in accordance with the number of cathodes that are being driven. In equilibrium, the common capacitance is charged as much during the cathode input pulses as the coupling capacitors are discharged in toto during conduction of the device.

United States Patent [1 1 Lee [ 1 Apr. 2, 1974 SELF-REGULATED DRIVE APPARATUS FOR DISPLAY SYSTEMS [75] Inventor: James Y. Lee, New Brunswick, NJ.

[73] Assignee: Burroughs Corporation, Detroit,

Mich.

221 Filed: Nov. 16, 1972 21 Appl. No.: 306,993

[52] US. Cl 315/169 TV, 315/169 R, 315/846 [51] Int. Cl. H05b 43/00 [58] Field of Search 315/169 R, 169 TV, 173,

[56] References Cited UNITED STATES PATENTS 11/1968 Myers et a1 315/169 TV 3/1969 Borkan 315/169 TV [57] ABSTRACT Apparatus addressing multi-position display devices directly from semiconductor integrated circuits and driving the display electrodes via passive coupling without exceeding the limitation on voltage excursions for the outputs of such circuits. The apparatus develops and applied to threshold-responsive display devices, including gas discharge display tubes or panels, only the amount of bias that is needed for operation of the device by the signal potentials that are available. Bias potential for the display electrodes is accumulated in steps until the device fires, and then is automatically adjusted each cycle in relation to the number of display electrodes that are being operated.

The input signals are applied to one set of electrodes through coupling capacitors which are charged from a common bias capacitance during blanking intervals between the input pulses and are at least partially discharged during operation of the device. The bias potential for those electrodes is developed across the common capacitance by a regulator passively coupled to the cathodes by series feedback resistors which add charge to the bias capacitance in accordance with the number of cathodes that are being driven. In equilibrium, the common capacitance is 'charged as much during the cathode input pulses as the coupling capacitors are discharged in toto during conduction of the device.

9 Claims, 2 Drawing Figures SELF-REGULATED DRIVE APPARATUS FOR DISPLAY SYSTEMS BACKGROUND OF THE INVENTION This invention relates to apparatus for operating multiple-digit display devices which are thresholdresponsive to selection signals applied to them, including gas discharge display tubes and display panels.

More particularly, the invention relates to apparatus for addressing multiple-position gas discharge devices directly from semiconductor integrated circuits and the like and to driving the display elements thereof via passive coupling without-exceeding the voltage limitations on the circuits that are being employed.

Various segmented-electrode display devices have been developed in recent years for use as readout indicators for-electronic calculators and the like. One such device is the PANAPLEX panel display which is a multiple-position gas discharge device having a plurality of segmented display cathodes and associated anode electrodes located in a common envelope. Such display panels usually include several groups of cathode segments, with like segments of the different groups being interconnected, and an anode associated with each group of cathode segments.

In a recently developed version of the PANAPLEX panel display, the cathode elements are deposited or formed along the front surface of an insulating base plate and planar anode electrodes are spaced closely above the cathodes. A relatively large difference in potential must be applied between an anode and one or more cathodes to initially ionize a display position, and a lower potential will re-ionize or sustain the discharge. It has been discovered that if the anodes and the cathodes thereof are sufficiently biased toward conduction that they can be operated by signals which do not exceed the maximum voltage excursions that are allowed on the outputs of some metal-oxide semiconductor (MOS) integrated circuits.

Such MOS integrated circuits are available in the form of calculator circuits, decoder circuits, counters, registers, and the like. Substantial economies would be obtained if the display positions of such devices could be addressed and driven directly from such MOS circuits with little intermediate amplifying or coupling circuitry being required at their interface.

A condition precedent to the use of such low voltage signals is that the electrodes of the device be biased toward discharge as much as possible without sustaining the discharge after selection signals are removed from them. Higher voltages also have to be applied across the electrodes for initial ionization than for reionization, without violating the allowable voltage ratings on the MOS circuits or other low voltage signal source.

The total potential required for initially ionizing and for re-ionizing or sustaining the display in gas discharge panels, however, varies with the temperature and the gas pressure about the electrodes. It also can vary significantly among tubes or panels of the same general type. A set of bias voltages provided to allow the electrodes of one lot of such devices to be addressed or driven by the smallest possible signal voltages may not be sufficient or may exceed the potential required to operate another lot of tubes of the same type.

SUMMARY OF THE INVENTION Accordingly, an object of the invention is to simplify and reduce the cost of addressing and driving multipleposition display devices or panels.

Another object is to address and drive the display positions of gas discharge devices via passive coupling from semiconductor integrated circuits or the like without exceeding voltage limitations on the circuits.

A further object of the invention is to regulate the drive voltage applied to multiple-position display devices without requiring switches or other active elements for controlling feedback from the drive circuits thereof.

The invention provides self-regulating drive apparatus for segmented electrode display devices which includes input capacitance elements and feedback resistances passively coupled to the display segments and a common biasing circuit for the segments. Bias potential for the cathodes is developed across a common charged which is cahrged by a current amplifier responsive to feedback of a portion of the drive currents available for the segments during the display period. The common capacitance is repetitively charged until the device fires.

The input coupling capacitances are at least partially discharged when the display conducts and are recharged from the common capacitance during blanking intervals between input pulses. This bias potential, and consequently the operating voltage, adjust automatically to compensate for differences in the potential thresholds of different units and for variations caused by changes in temperature or mode of operation.

DESCRIPTION OF THE DRAWINGS Other advantages and features of the invention are made clear in the following description, relating to the attached drawings, wherein:

FIG. 1 is an enlarged, exploded view of a segmentedelectrode display panel shown in perspective; and

FIG. 2 is an electrical schematic circuit diagram of a drive and bias system for such a panel display.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The display panels described herein are thin, flat, sheet-like members which may be of substantially any desired shape and size, and may include substantially any number of character display positions. The panels may include any suitable ionizable gas such as neon, argon, xenon, etc., either singly or in combination, with a vapor of a metal such as mercury usually included in the gas to minimize cathode sputtering. A wide range of gas pressures may be used, for example, from about 20 to about 250 Torr at ambient temperature with about to Torr being a commonly used pressure range.

Referring to FIGS. 1-3, a display device 10 embodying the invention includes an insulating base plate 20 of glass, ceramic, or the like, with an inexpensive glass being suitable and preferred. A plurality of conductive connectors or runs 30A to 30G are formed on the top surface of the insulating plate 20. The runs 30 are parallel to each other and are aligned with the horizontal axis of the base plate. Seven such runs 30A to 30G are shown; however, more or fewer may be provided, the number being determined by the total number and type of characters to be displayed. The runs 30 may be formed by an evaporation process, a silk-screen process, an electroless plating process or the like, or they may be discrete strips of metal, heat-sealed, and plasma-sprayed, or otherwise secured to the insulating plate 20. A silk-screen printing process is particularly suitable because it is fast, efficient, and reproducible.

A second thin layer 40 of insulating material such as glass or ceramic is formed on the conductive runs 30, preferably by a silk-screen process, and the second layer 40 includes a plurality of groups of vias or apertures 50A to 506, each aperture exposing one of the runs 30A to 30G. Thus, each group of apertures includes apertures 50A which aperture run 30A, aperture 508 which exposes run 30B, aperture 50C which exposes run 30C, etc. Four such groups of apertures are illustrated.

Panel includes a group of cathode electrodes 60 (A to G) for each group of apertures 50. The cathodes are generally elongated bars or segments, and they are usually arrayed in a figure 8 pattern, as is well known in the art. The cathodes 60 maybe formed on insulating layer 40 by means of a silk-screen process using a conductive paste such as palladium-gold, platinum-gold, palladium-silver, or the like. Each cathode element is in contact with one of the runs exposed by one of the apertures or vias 50, and it substantially fills the aperture 50 and covers a portion of layer to achieve the desired shape and size.

The cathodes 60A, 60B, etc. may also be formed of discrete strips of metal, preferably brazed to a conductive run 30 by means of a mass of brazing material deposited in each of the apertures in the insulating layer 40. The brazing material itself may be deposited by a silk-screen process. One suitable brazing material is a gold-germanium substance known as FORMON and sold by E. l. DuPont de Nemours & Co. The cathodes may also be formed in any other suitable manner such as by electrolytic or electroless plating of nickel or the like or by are plasma spraying through a suitable mask.

Thus, cathodes are preferably thin, flat members which do not project to any significant extent above the top surface of insulating layer 40.

Panel 10 includes anode electrodes for the groups of cathode electrodes 60. The anode electrodes comprise thin, transparent conductive films of gold, NESA, or the like formed on the lower surface of the panel face plate or viewing plate which is made of glass.

The anode films are of the order of a few Angstroms thick and, in effect, are coplanar with the bottom surface 95 of the face plate. Thus, the anodes, for all practical purposes, do not project into the gas discharge space in the panel. The anode films are generally rectangular in shape, or are otherwise shaped, depending on the orientation of the cathodes. Anodes 90 are dimensioned and positioned so that they overlay the total area defined by the associated group of cathode electrodes. If desired, each anode 90 may be somewhat narrower and shorter than the area defined by its cathodes as shown, but in any case, the anode must overlay and be in operative relation with a sufficient portion of each of its cathodes. Other suitable anode shapes may be employed, depending upon the character and symbol configuration of the cathodes to be operated.

Preferably, the spacing between each anode 90 and its group of cathodes 60 should be of the order of 20 to 25 mils, and the spacing between each anode and the adjacent group of cathodes should be of the order of 30 to 40 mils. With this relationship at the usual pressure range, each anode is in a favorable operating position with respect to its own cathodes, but is sufficiently remote from adjacent groups of cathodes so that the panel may be operated over a suitably wide range of potentials without developing cross-talk between adjacent groups of electrodes. Another factor tending to prevent cross-talk is the location of the anodesin substantially coplanar relation with the surface of the glass cover plate and not projecting into the gas space in which cathode glow takes place.

Another advantage of the close spacing of each anode to its'group of cathodes, thus providing a thin volume of gas, is that metastable-state atoms produced in the gas during discharge diffuse to, and are readily neutralized at, these closely spaced surfaces. In addition, excited or charged particles are readily swept out through the anode-cathode circuit path. The tendency for cross-talk to develop is minimized by these two factors.

The top glass cover plate 100 is of substantially the same length as the insulating layer 40 and the bottom plate 20, and it is spaced from the base plate 20 by a rectangular glass frame 1 10 which is disposed between the top glass plate 100 and the insulating layer 40. Frame may be an integral part of the top and/or bottom plates. The rectangular frame serves thus to provide the desired spacing between each anode and its associated group of cathode electrodes. The top glass plate 100 is also preferably slightly wider than the insulating layer 40 and base plate 20 so that one edge, such as the upper edge, extends beyond the remainder of the panel and is accessible topermit the connection of leads to each of the anode films 90. The three glass members 20, 100, and 110 are sealed together in any suitable manner, for example, by means of a seal 120 formed of a glass frit or the like.

Connection to the runs 30 may be made by means of L-shaped pins or contacts 144 which are embedded in the seal 120 at one or both ends of the panel, for example.

The panel 10 can be filled with the desired gas atmosphere through a tubulation secured to the base plate 20 and communicating with the interior of the panel through a hole 154 in plate 20 and layer 40 and, generally, mercury is introduced from a glass capsule (not shown) held in the tubulation and suitably processed at the desired state in the assembly process.

These panel-type segment display devices include a plurality of groups of elongated bars or segments arrayed in a pattern so that each group can be selectively energized to display a character. For reasons of economy, corresponding electrodes in each group, usually cathodes, have a common conductor/The anodes are separately energizable and the panel is operated in a multiplex mode of operation. In this mode of operation, operating potential is applied to selected cathode conductors at time t and thus to selected cathode segments, and the first anode is energized and a first character is displayed by the energized cathode segments in the first group. At time t,,, operating potential is applied to the same or other cathode conductors and to the second anode and a second character is displayed by the second group of cathodes. This operation is carried out for each character position, and it is repeated continually along the entire display panel at a suitable frequency so that stationary but changeable characters can be displayed.

The display system of FIG. 2 is an embodiment of the self-regulated drive apparatus for multiple-position display devices having several groups of cathode segments or elements 60 (A-l-l) interconnected by cathode conductors 30 (A-l-l), and a plurality of asosciated anode electrodes 90. A digit sequencer 200 is connected to the anode electrodes by conductors 190 and contains a digit selecting or addressing switch for energizing the anodes independently and in a suitable sequence, either successively or alternately. Each of the anodes 90 is also coupled to a bias voltage bus 185 by a resistor 195. The anode bias bus 185 is connected to voltage reference terminal 180, and the resistors 195 are effective to bias the anodes near the required operating potential. Anode bias resistors 195 both pull the anodes down to the bias potential on bus 185, after they have been energized, and prevent the bias potential on the unselected anodes from being pulled down too far capacitively as the cathodes in the device are pulsed to operate with the selected anode. Digit sequencer 200 may comprise a counter or shift register and may be incorporated in a common integrated circuit with data source 300 in some applications.

The cathode portion of the system includes input capacitors 210 connected to the cathode connectors 30 (A-H) by input conductors 205 and to data signal source 300 by conductors 215 at circuit junctions 212. The input conductors 215 are also connected at circuit junctions 212 to pull-down resistors 220, the other ends of which are connected in common to voltage reference terminal 225. Data signal source 200 may be a metal oxide semiconductor (MOS) integrated circuit such as a calculator unit or the like and may incorporate digit sequencer 200. Such MOS integrated circuits often provide an open-circuit or open-drain output for negative-going signals. Resistors 200 connected to input conductors 215 cause negative-going signal excursions to be applied through input capacitors 210 to cathode conductors 205 when the inputs become opencircuited.

Cathode conductors 205 are also each coupled by a series-connected resistor 230 or 232 to a common feedback bus 235 which is connected to a current amplifier comprising, in this embodiment, PNP transistors 240 and 250 connected in the Darlington amplifier configuration. Feedback bus 235 is connected to the base electrode of transistor 240. The emitter electrode of transistor 240 is connected to the base electrode of transistor 250, which is also coupled to its collector electrode by resistor 245 to shunt leakage current from transistor 240 around transistor 250. The collector electrodes of both transistors are connected in common to a voltage reference terminal 260. The emitter electrode of transistor 250 is coupled by resistor 255 to a cathode biasing or restoring bus 265, to which is connected a relatively large capacitor 270 in electrical parallel with a resistor 275 and a reverse-biased Zener diode 280 to a ground terminal. Capacitor 270 stores or integrates the bias developed on voltage bus 265, and resistor 275 prevents a voltage from being developed on bis bus 265 by leakage current through transistor 250 when no display signals are present on the display electrodes. Zener diode 280 limits the maximum voltage that will be established on bus 265 during initial ionization of a display device having a particularly high ionization threshold, as at low temperatures, to prevent excessive currents. A resistor 242 in series with a pair of diodes 244 is connected between the base electrode of transistor 240 and bus 265 to establish the current gain of the amplifier and to compensate for the baseemitter thresholds of the transistors.

The bias-restoring bus 265 is also coupled to cathode conductors 205 by diodes 285. These diodes 285 become reverse-biased upon the application of negativegoing signals on input terminals 215 which are coupled through input capacitors 210 to the cathodes. Diodes 285 are forward-biased at the end of each cathode input signal and during blanking intervals between them. A Darlington current amplifier comprising transistors 240 and 250 is illustrated in the preferred embodiment to provide a substantially constant high current gain. A single transistor having an adequately large current gain may be used instead.

The system of FIG. 2 provides feedback from the input signals received by the cathodes to a circuit which regulates the bias on them without requiring the use of feedback switches between the cathodes and the regulating circuit. The feedback from all of the cathodes is coupled to the current amplifier comprising transistors 240 and 250 by series-connected resistors 230 and 232 which are high in value compared to the internal resistance of device 10 when it is conducting. Although no switches are provided to control the feedback current, it will be considerably higher during ionization delay when the full input signal appears across sensing resistors 230, 232 than it is after the display device fires. Conduction of feedback current while the display device is conducting causes cathode bias capacitor 270 to be charged to a voltage based on the operating voltage or tube drop of the device and conduction of feedback current after the device has discharged input capacitors 210 and ceased conducting, bases the cathode bias on the off-cathode voltage level. It has been discovered that this allows devices of varied characteristics to be operated with smaller signal swings than heretofore.

Each cathode circuit comprises a level-shifting input capacitor 210, a resistor 220, a feedback resistor 230 or 232, and a restoring diode 285. Resistor 220 is coupled to common reference terminal 225 as a pull-down resistor for open-circuit or open-drain cathode inputs from metal oxide semiconductor circuits, for example. Assume that the system is first turned on and that the voltage on the bias-restoring bus is at an arbitrary potential Va and that no voltage appears across input capacitor 210. Anode digit pulses are applied sequentially to anodes 90. The first negative signal excursions received for selected ones of the cathodes will couple through the corresponding input capacitors 210 and develop current through the corresponding feedback resistors 230 and 232 which is amplified by the transistor current amplifier, thus charging capacitor 270 on the cathode-restoring bus to Va-AV. This voltage will be maintained by capacitor 270 until after the display device fires or until it is increased again by the feedback current amplifier.

When the cathode input signals return from their negative excursion, the selected cathode input capacitors 210 will be charged to approximately AV through the corresponding diodes 285 coupling them to the bias-restoring bus 265. During the next cycle the biasrestoring bus 265 will be charged to Va2AV, assuming the same number of cathodes are energized, and the selected cathode input. capacitors 210 will be charged to 2 V. This will continue until the signal and bias voltage between the selected anode and cathode exceeds the ionization potential.

When the bias voltage on the cathode input capacitors 210 is sufficient that the display device fires at the selected display position upon superposition of the input signals upon the bias, the system is in equilibrium. With display devices of the gas discharge panel type, the bias voltage on restoring bus 265 will rise to a high value to cause initial ionization, after which it will settle to a lesser value to support re-ionization of the successive display positions in the device. The Zener diode 280 is coupled from the bias-restoring bus 265 to ground to limit the maximum excursion of the bias voltage to protect the system from over-voltage conditions.

In normal operation, bias-restoring bus 265 for the cathodes is self-regulated to the voltage drop across the display device and to the off-cathode voltage level. Once ionized, the display device conducts during each display period and substantially discharges the corresponding cathode input capacitors 210 up to the voltage at which the device no longer conducts. The presence of blanking intervals between successive cathode signals is desirable to allow for the recharging of the cathode level-shifting capacitors 210 from bias-storing capacitor 270 at the end or at the beginning of each digit display period.

During the blanking intervals, the cathode levelshifting capacitors 210 are all charged from bias capacitor 270 to the voltage on biasing bus 265 via the restoring diodes 285. The capacitors 210 are then ready to level-shift the data signals down to the cathodes being selected. When selected, both the voltage on the energized cathodes during the re-ionization delay time and the panel sustaining voltage on them are more negative than the restoring bus 265. The feedback or sensing resistors 230 and 232 thus conduct current from the base electrode of transistor 240 while the cathodes are energized. This base current is amplified by the current amplifier to charge bias integrating capacitor 270 which will re-charge the several level-shifting capacitors 210 during the next cathode blanking interval. The charges lost by the input capacitors 210 of the selected cathodes will activate the corresponding sensing resistors 230, 232 to charge bias capacitor 270 via the current amplifier by the same amount.

The amplifier provided for charging capacitor 270 must have sufficient current gain to charge the capacitor equal to the amount that cathode input capacitors 210 are discharged during operation of the device. This allowsthe input capacitors 210 to be recharged from the bias-storing capacitor 270 during the blanking interval between digits without any net effect on the established bias level. The required current amplification is determined by the ratio of display current to feedback current which is determined by the tube drop across the device and the value of feedback resistors 230, 232. If the feedback resistors 230, 232 are reduced, then the current amplification of the charging circuit for bias capacitor 270 may also be reduced, but

the display current and display brightness will be reduced also. A single transistor amplifier of adequate current gain and one compensation diode may be substituted for the amplifier comprising transistors 240 and 250 and diodes 244, if desired.

The feedback currents from the cathode sensing resistors 230, 232 are summed at the current amplifier to which they are all connected in common. The regulating circuit charges the bias capacitor 270 proportionally and establishes an average cathode bias for the cathodes. This bias will be sufficient for operation of most combinations of the cathode segments for display but may result in long ionization delays if only a few segments are energized.

In the preferred embodiment of FIG. 2, two groups of the display cathodes are connected to feedback resistors 232 of lower value than feedback resistors 230 of all the other cathodes, as indicated. These two cathode segments are frequently operated as a pair for the display of the numeral 1 or are used as part of the numeral 7 or other symbol, without additional cathodes being driven. Initial ionization and re-ionization of the device is slower with only a few of the cathodes being driven. The low resistance feedback resistors 232 couple a greater portion of the input current back to the current amplifier and thus charge the bias capacitor 270 to a higher voltage level for assuring that such cathodes will suitably ionize the device when selected. Such a low value feedback resistance also might be provided for sensing any other display segments that are operated alone or as pairs in the display of a numeral or symbol.

Although the preferred embodiment of the invention has been described in detail,-it should be understood that the present disclosure has been made by .way of example only. Many modifications and variations of the invention are possible in light of the above teachings. It is, therefore, to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically disclosed.

What I claim is:

l. A system for operating a multiple-position display device having a plurality of groups of cathode electrodes and an anode associated with each group, corresponding cathodes of the different groups being interconnected by a single common conductor, there thus being a single common conductor for each corresponding cathode electrode in each of said groups, said system comprising means connected to said anode electrodes for applying positive-going signal voltages to each of the anodes individually and sequentially,

a first bus connected through separate diodes to each common conductor,

a second bus connected through a resistive path to each common conductor and sensing the number of cathodes energized at each position,

a third bus connected between a voltage supply and through a separate resistive-capacitive path to each common conductor,

a data source having an output connected to each resistive-capacitive path and adapted to couple input signals to the cathode electrodes to selectively energize said cathodes, and

a current amplifier connected between said first and second buses, with said second bus being connected to the input of said current amplifier, the

output of said current amplifier being connected to a charging capacitor and to said first bus and serving to sense the total number of cathodes energized at each position at any instant in time.

2. The system defined in claim 1 wherein said current amplifier includes semiconductor amplifier means having its input connected to said second bus and its output connected to said first bus, and including, across said semiconductor means, the parallel combination of a resistor, a capacitor, and a diode.

3. The system defined in claim 1 wherein said capacitor in the circuit of said current amplifier is adapted to be charged thereby, said capacitor being coupled to said first bus and adapted to pre-bias the cathode electrodes toward discharge potential.

4. The system defined in claim 3 wherein the resistance of said resistive path has a larger value than the operating impedance in said device and said resistive paths are coupled between said cathodes and said current amplifier for charging said capacitor both during and after any delay which occurs in activating the device which occurs.

5. The apparatus of claim 3 wherein the resistance of the resistive paths associated with selected cathode electrodes are lower in value than the others in order to increase the rate of charging of the said capacitor and to achieve larger pre-bias potential to ensure activation of said selected cathodes.

6. The system of claim 1 wherein blanking intervals are provided between said input signals to said cathode electrodes, during which intervals said capacitor receives charge from said resistive-capacitive path.

7. The system of claim 6 wherein said capacitor is coupled to the cathode electrodes by unidirectionally conductive means which conduct during the blanking intervals to charge the capacitor in said resistivecapacitive paths.

8. The system of claim 1 wherein the anodes are biased toward energization from a common reference potential terminal to reduce the amplitude of the signal voltages that are necessary to activate the device.

9. The system of claim 8 wherein the anodes are coupled through resistors to said common reference potential. 

1. A system for operating a multiple-position display devicE having a plurality of groups of cathode electrodes and an anode associated with each group, corresponding cathodes of the different groups being interconnected by a single common conductor, there thus being a single common conductor for each corresponding cathode electrode in each of said groups, said system comprising means connected to said anode electrodes for applying positivegoing signal voltages to each of the anodes individually and sequentially, a first bus connected through separate diodes to each common conductor, a second bus connected through a resistive path to each common conductor and sensing the number of cathodes energized at each position, a third bus connected between a voltage supply and through a separate resistive-capacitive path to each common conductor, a data source having an output connected to each resistivecapacitive path and adapted to couple input signals to the cathode electrodes to selectively energize said cathodes, and a current amplifier connected between said first and second buses, with said second bus being connected to the input of said current amplifier, the output of said current amplifier being connected to a charging capacitor and to said first bus and serving to sense the total number of cathodes energized at each position at any instant in time.
 2. The system defined in claim 1 wherein said current amplifier includes semiconductor amplifier means having its input connected to said second bus and its output connected to said first bus, and including, across said semiconductor means, the parallel combination of a resistor, a capacitor, and a diode.
 3. The system defined in claim 1 wherein said capacitor in the circuit of said current amplifier is adapted to be charged thereby, said capacitor being coupled to said first bus and adapted to pre-bias the cathode electrodes toward discharge potential.
 4. The system defined in claim 3 wherein the resistance of said resistive path has a larger value than the operating impedance in said device and said resistive paths are coupled between said cathodes and said current amplifier for charging said capacitor both during and after any delay which occurs in activating the device which occurs.
 5. The apparatus of claim 3 wherein the resistance of the resistive paths associated with selected cathode electrodes are lower in value than the others in order to increase the rate of charging of the said capacitor and to achieve larger pre-bias potential to ensure activation of said selected cathodes.
 6. The system of claim 1 wherein blanking intervals are provided between said input signals to said cathode electrodes, during which intervals said capacitor receives charge from said resistive-capacitive path.
 7. The system of claim 6 wherein said capacitor is coupled to the cathode electrodes by unidirectionally conductive means which conduct during the blanking intervals to charge the capacitor in said resistive-capacitive paths.
 8. The system of claim 1 wherein the anodes are biased toward energization from a common reference potential terminal to reduce the amplitude of the signal voltages that are necessary to activate the device.
 9. The system of claim 8 wherein the anodes are coupled through resistors to said common reference potential. 